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24 July 2023

Should all New Houses be Modelled for Thermal Performance? – Part 2

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Last month I wrote questioning the proposal presented in BRANZ’s magazine ‘BUILD’ (issue #193, December 2022), which included the statement that; “The H1/AS1 schedule method of determining insulation needs will likely be withdrawn, and modelling will be required for either embodied carbon or energy use or potentially both.”, and goes on to further state that; “Computer modelling for things like energy use and carbon footprint will eventually be the norm for new buildings, including houses.”

In the ‘DISQUS’ comments section of my June 2023 EBOSS writing, and in other discussions away from this blog, there was comment from colleagues on both the passive thermal/embodied carbon performance linkage, and the withdrawal of the Schedule Method for demonstrating NZBC/H1 compliance. I, (and hopefully most readers), will agree that there is no direct correlation that if a design maximises passive thermal performance then it will also perform positively to minimise embodied and operational carbon content. This lack of correlation is the same as trusting that good passive performance will be equated with good structural performance. It is entirely possible to design a dwelling which is dangerously non-complying as a structure, but has superb passive thermal performance.

Over the years practitioners have expressed a preference for the Calculation Method option to demonstrate compliance with NZBC-H1. As most realise, this is not a modelling method but is in essence the Schedule Method applied twice. First the calculations are completed for the proposed house using the preferred Construction R-values, and then repeated substituting the Schedule Method R-values as a reference. If the heat loss for the proposed is less than that for the reference building then the dwelling complies with NZBC-H1. If the Schedule Method is abandoned in favour of only Modelling, then the opportunity for the design flexibility and convenience of the Calculation Method will be lost.

If the proposal is to only use modelling, (and it will need to apply to each dwelling separately to make any sort of sense), as the method to show NZBC compliance, then there is likely to be a temptation to link the thermal and carbon analyses together as one — after all they are both seeking the same sustainability target. I don’t see how this can happen in any practical and sensible way. It would be the same as trying to enmesh the structural and thermal performance aspects of the NZ Building Code into one combined compliance, rather than having the two compliances as at present. Of course they both have to be considered together during the design and documentation process but this is in tandem as a cyclical feedback path. There are a multitude of these complex inter-relationships to be resolved in order to attempt to satisfy all aspects of a building’s design brief. Because the overall result must always be a compromise, what is the benefit of requiring individual precision calculations for only a few aspects? In my Detailed Blog of December 2014 ‘Insulation, Thermal Mass and Glazing: The Juggling Game’, I discuss these multifaceted inter-relationships.

The fundamental problem with trying to obtain the ideal passive thermal performance of a real building is that it exists in a dynamic environment, rather than in a laboratory with an assumed static setting and applying standardised fixed parameters and assumptions. (Although I fully agree that it is right and proper for scientists to use this process, otherwise, results would not be comparable.) The environment in which any actual dwelling is located is subject to varying diurnal conditional, and seasonal changes through the year before consideration is given to equally important diverse factors such as the local typography, adjacent buildings, and other shading features, etc. Daily weather is also a substantial influence but much too fickle to contemplate incorporating into any analysis, just as curtains are. This is why any discrete modelling result for NZBC compliance cannot be applied beyond its individual residence. To do is to rapidly return to the Schedule Method. Even in an apartment building each dwelling has a different passive thermal performance — those on the east face have completely different conditions to those on the west even though the plans, elevations, materials etc., are the same.

The same goes for embodied and operational carbon. In the laboratory the scientists can establish the theoretical carbon content of ideal materials, components and construction systems etc., and also of appliances and equipment involved with the operational carbon of a building’s lifetime, but would the results be practical data for direct application to daily ‘in-use’ situations? In the construction world the variety of materials, products and equipment, etc. is limited to the reality of commercial markets where the ideal item is not available or not in the ideal sizes etc., unless of course a much more expensive and time consuming customised supply is ordered.

In last month’s blog post I referred to the AccuRate thermal simulation software which EECA brought to New Zealand from Australia, after a world-wide search, and adapted it, as AccuRateNZ, for local conditions to be the software for their HERS (Home Energy Rating Scheme). It was put in abeyance soon after release as political ideology changed and it was too soon for the public to be interested in such a detailed thermal analysis. Even so, the analysis is still valid and, as I recently confirmed with MBIE, acceptable by Councils to show NZBC-H1 compliance by the modelling method. Compliance is shown by calculating the Building Performance Index (BPI). NZBC-H1 Verification Method compliance can be shown using AccuRateNZ by simply substituting the Schedule Method R-values, as for the Calculation Method, and re-calculating. The minimum requirements of the BPI regulation are in the background for determining the R-values stated in the Schedule Method.

Unlike ALF and Passivhaus, AccuRateNZ analyses the thermal performance of a house when only passive (solar and environmental etc) heating and cooling gains and losses are considered (each side of the standard temperature comfort range). These are calculated at hourly intervals throughout a year for each space within the building. These volumes include any sub-floor, garage, attic, and ancillary spaces within the exterior envelope as these all interact in three dimensions with their adjoining volumes. The calculations result in 8,760 (24 hours for 365 days), temperatures for each of up to 50 volumes within the dwelling. The analysis is also appropriate for terrace and multi-storey apartment blocks.

The temperature figures are kept within the software and used to calculate the published total annual energy requirements per square metre for both the entire building and then just the habitable spaces. The temperature results can be extracted from the computer but because they, (as with all modelling), have been calculated using a standard set of occupant behaviours, (and daily weather is unpredictable), they cannot be used to state the precise temperature for a particular room for a specific day and hour. Even so, they give a good enough result to be used with care to explore and design any contemplated heating and cooling system. If wished there is the opportunity for AccuRateNZ to move on with these results to explore a wide variety of options for heating systems for each room.

The energy shortfall from the passive thermal performance is published in three parts. The heating component is straight-forward, but the cooling calculations are divided into two — first the Sensible Cooling which is the energy to lower the temperature to a maximum of 25°, and the second the Latent Energy required to dehumidify the air because as air cools its humidity increases. When a room’s temperature is shown to be above 25° the software goes back to the beginning of the hour, opens all the window sashes and door and then re-runs the calculations. If the result is below 25° then zero deficit is recorded but if still above the threshold then the original temperature is recorded.

If it is insisted that only modelling can be used for NZBC-H1 compliance then AccuRateNZ is a viable option as EECA accepted it in the 2000s for the HERS analysis. Only a dusting off, and perhaps the addition of some of Australia’s bells-and-whistles, would be required. I understand it is still used in a few large practices and also by BRANZ.

As I suggested in my June 2023 Detailed Blog, I encourage all readers to give serious thought to the possible withdrawal of the Schedule Method, and as a consequence the Calculation Method, from NZBC-H1 before it becomes a reality. Personally I am of the opinion that the Schedule Method must be kept as an adequate and simple path for all designers to use without bringing in specialist skills.

Through EcoRate Ltd – Architect I provide objective independent passive solar thermal performance analysis, using AccuRateNZ, and advice on sustainability matters, to architects, designers, builders, manufacturers, and others in the construction industry, included those proposing to build a new home.

For more information feel free to contact Keith at EcoRate Ltd on 021 890 251, [email protected], or our website

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